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Background: GARP transcription factors perform critical roles in plant development and response to environmental stimulus, especially in the phosphorus (P) and nitrogen (N) sensing and uptake. Spirodela polyrhiza (giant duckweed) is widely used for phytoremediation and biomass production due to its rapid growth and efficient N and P removal capacities. However, there has not yet been a comprehensive analysis of the GRAP gene family in S. polyrhiza. Results: We conducted a comprehensive study of GRAP superfamily genes in S. polyrhiza. First, we investigated 35 SpGARP genes which have been classified into three groups based on their gene structures, conserved motifs, and phylogenetic relationship. Then, we identified the duplication events, performed the synteny analysis, and calculated the Ka/Ks ratio in these SpGARP genes. The regulatory and co-expression networks of SpGARPs were further constructed using cis-acting element analysis and weighted correlation network analysis (WGCNA). Finally, the expression pattern of SpGARP genes were analyzed using RNA-seq data and qRT-PCR, and several NIGT1 transcription factors were found to be involved in both N and P starvation responses. Conclusions: The study provides insight into the evolution and function of GARP superfamily in S. polyrhiza, and lays the foundation for the further functional verification of SpGARP genes. [ABSTRACT FROM AUTHOR]

Duckweeds is the most promising nonfood feedstock for sustainable bioenergy production. Mixotrophy is the best growth mode compared to photoautotrophy and heterotrophy for biomass and starch accumulation in duckweed. This work investigated the impacts of light intensities (3,000, 6,000, 9,000, 12,000, 15,000, and 20,000 lux) and exogenous glucose concentrations (5, 10, 15, 20, 30, and 40 g L−1) on the biomass and starch accumulation of mixotrophic duckweed. The results showed that increasing light intensity and the initially applied concentration of glucose enhanced both growth and starch content. However, high concentrations of glucose (15–40 g L−1) resulted in a higher biomass yield but concurrently greatly inhibited photosynthesis. Moreover, excess carbon was redirected into starch biosynthesis under high glucose. The maximum starch yield of 655.4 g m−2 was obtained at 40 g L−1 glucose, with a maximum starch content of 72.7% of dry biomass and a biomass yield of 1024.6 g m−2. The optimum light intensity for starch accumulation was 20,000 lux, with a maximum starch yield of 236.2 g m−2. These results indicated that high organic carbon levels are a robust way to promote starch production in mixotrophic duckweed. • High glucose levels and light intensities boosted biomass and starch production. • Higher glucose levels provided a higher biomass while inhibiting photosynthesis. • More carbon was allocated to starch biosynthesis with higher glucose levels. • 20,000 lux was the optimum light condition for duckweed starch accumulation. • The maximum starch yield of 95.7 t/ha/year was generated at 40 g/L of glucose. [ABSTRACT FROM AUTHOR]

Duckweeds contain high levels of starch and are environmentally sustainable and economically viable feedstock for biofuel production. Here, the biomass and starch yield of three duckweed species under three different nutrient-limited conditions were analyzed to investigate the possible ways of further increasing the efficiency of starch production. The results showed that sulfur limitation resulted in the highest starch yield, which was 42% and 73% higher than in nitrogen or phosphorus limitation conditions, respectively. The high yield of sulfur-limited duckweed is largely due to the combinations of little effects on biomass and high accumulations of starch. Although nitrogen limitation led to higher starch content (67.4%), it severely reduced biomass production. Besides, this work revealed the mechanism of starch accumulation induced by sulfur limitation in duckweed based on transcriptomic analysis. In summary, sulfur limitation is a practical approach to increase starch yields in duckweed without affecting growth or biomass production. • Sulfur limitation induces starch production without inhibiting growth of duckweed. • Starch content was as high as 56.2% of dry weight under sulfur limitation. • Sulfur limitation results in the highest starch yield in three duckweed species. • SDI1 and bZIP17 were identified for improving starch yield by genetic engineering. • The mechanism of sulfur limitation-induced starch accumulation was elucidated. [ABSTRACT FROM AUTHOR]